Many functions of modern devices in automotive, consumer and industrial applications, such as computer technology, mobile communications technology, converting electrical energy and driving an electric motor or an electric machine, rely on semiconductor devices, in particular semiconductor transistors such as field-effect transistors (FETs), for example MOSFETs (Metal Oxide Semiconductor Field-Effect Transistors) and IGBTs (Insulated-Gate Bipolar Transistors), and BJTs (Bipolar Junction Transistors).
It is often desirable that rectifying semiconductor devices such as diodes and IGBTs have a sufficiently high blocking capability. Accordingly, their rectifying pn-junction or pn-junctions are often desired to withstand sufficiently high reverse voltages. Unfavorable dimensioning may result in avalanche generation close to or at points where the rectifying pn-junctions come to or near a surface. Accordingly, blocking capability may be reduced to values well below the value of the bulk breakthrough field strength of the semiconductor material.
To reduce the intensity of the electric fields near the edge of a rectifying junction (e.g. pn-junction), high voltage semiconductor devices may include an edge termination structure in a peripheral area arranged around an active area with the rectifying junction. An edge termination structure provides a transition region in which the high electric fields around the active area change gradually to the lower potential at the edge of the device. The edge termination structure may, for example, lower the field intensity around the termination region of the rectifying junction by spreading the electric field lines across the termination region.
Planar edge-termination structures such as field plates, guard-ring structures or channel stop region are arranged on or close to a main horizontal surface of the semiconductor device. Often a combination of several edge-termination structures is used. To achieve high blocking capability and stability, a comparatively large peripheral area is typically required when planar edge-termination structures are used. Furthermore, the size of the peripheral area typically rises with rated blocking voltage. For example, for a rated blocking voltage of 600 V one or more field-plates are used with a horizontal extension of the resulting edge-termination system of at least about 150 μm is typically required. For a rated blocking voltage of about 6.5 kV the horizontal extension of the edge-termination system using field plates is typically larger than about 2 mm. Accordingly, the fraction of the active area used for switching and/or controlling the load current is significantly reduced, and thus the costs per chip or die increased. Furthermore, forming these structures is often associated with increased processing requirements.
Different thereto, vertical edge-termination structures, also known as mesa edge-termination structures, typically require less space. For example, a circumferential vertical trench filled with an insulating or a semi-insulating material may be used as edge-termination structure. However, for higher rated blocking voltages of 600 V or more, the desired horizontal width of a circumferential vertical trench filled with an insulating material is comparatively large. This may cause high mechanical stress. Furthermore, charges trapped in the insulating material may, in particular for bipolar semiconductor devices, result in increased switching losses. Depositing semi-insulating materials on vertical sidewalls of the circumferential vertical trench is, on the other hand, associated with increased processing requirements.
For these and other reasons there is a need for the embodiments disclosed in the present application.